1. Field of the Invention
The present invention relates to a semiconductor light emitting device, more particularly to a semiconductor light emitting device using a GaN-based material and a method for producing the semiconductor light emitting device. The present invention also relates to a method for forming a GaN based semiconductor layer.
2. Description of the Related Art
GaN has a wide bandgap and light emitting diodes (LEDs) using GaN have been therefore known as semiconductor light emitting devices emitting light in the region from blue to violet. Although semiconductor light emitting devices emitting blue light are prospective key devices in an optoelectronic field, there is a problem that it is very difficult to grow a GaN bulk crystal with high quality. Due to the problem, the research on the growth of GaN bulk crystals deals with the selection of a suitable substrate and on a method for the GaN crystal deposition.
In this connection, a conventional method has tried to use a single-crystal sapphire substrate for depositing the GaN layer thereon by applying the metal-organic chemical vapor deposition method (hereinafter referred to as the xe2x80x9cMOCVD methodxe2x80x9d). In this method, however, a difficulty lies in depositing a GaN layer of high-quality crystallinity. This difficulty is attributed to a great difference between lattice constants of the sapphire substrate and GaN (a particular difference between the lattice constants of the two is as great as about 16.1%), thereby developing crystal defects with a dislocation density as large as 108 to 1011/cm2 in the deposited GaN layer.
In recent years, a method has been proposed to cope with problems such as that described above. In the proposed method, a polycrystalline or amorphous crystalline AlN layer is deposited as a buffer layer between a sapphire substrate and a GaN layer in order to reduce the difference between the lattice constants of the single-sapphire substrate and the GaN layer, thereby enabling the deposition of a GaN layer of high-quality crystallinity. It is also disclosed that using a ZnO layer as a buffer layer enables the deposition of the GaN layer on, in addition to the single-crystal substrate, amorphous crystalline substrates such as a quartz glass substrate; and practical applications of this method are being developed (for example, Japanese Unexamined Patent Publication No. 8-139361).
Even in these methods of the related art which form the GaN layer by using a buffer layer, such as an AlN layer or a ZnO layer, on a substrate, the MOCVD method is a mainstream method being used to deposit the GaN layer.
Despite technical development, however, conventional semiconductor light emitting devices have the problems described below.
Specifically, the single-crystal sapphire substrate used hitherto predominantly as a substrate for a GaN layer increases the production costs because the substrate is higher-priced.
In addition, in both of the aforementioned methods of the related art, the MOCVD method is used. In the MOCVD method, however, a substrate needs to be heated to a high temperature of 1,000 to 1,200xc2x0 C. at the time of vapor deposition to make use of a thermal decomposition reaction for the crystal deposition. Such a high temperature causes the following problems. First, substrates that can be used for depositing a GaN layer thereon are limited to those having a high heat-resisting property. Second, because of the high temperature, a substrate receives a strong effect resulting from a difference between the coefficients of thermal expansion of the substrate and GaN. A particular example is a device in which the GaN layer is deposited on a single-crystal sapphire substrate. In this example, if the substrate on which a GaN layer is deposited at about 1,000xc2x0 C. is cooled from about 1,000xc2x0 C. to an ambient temperature, the substrate shrinks greater than the GaN layer due to a difference (about 34%) between coefficients of the single-crystal sapphire substrate and the GaN layer. This often causes distortions, cracks, and lattice defects in the GaN layer, which results in degradation of the crystal quality. As a result, it is difficult to obtain a device having a sufficient light emitting effect.
For the forgoing reasons, there is a need for a semiconductor light emitting device that comprises a GaN-based layer having an excellent crystallinity and a sufficient light emitting effect and that can be produced at a low cost. There is also need for a method for producing the semiconductor light emitting device and a method for forming a GaN-based semiconductor layer having an excellent crystallinity.
The semiconductor light emitting device according to the invention comprises: a glass or silicon substrate having a softening point of 800xc2x0 C. or less; a ZnO buffer layer provided on the glass substrate; and semiconductor structure including at least one light emitting layer made of a GaN-based semiconductor.
The light emitting layer is preferably formed by using an ECR-MBE method. The semiconductor light emitting device may further comprises an amorphous GaN-based semiconductor buffer layer between the ZnO buffer layer and the light emitting layer. The light emitting layer may be made of GaN semiconductor or InGaN semiconductor.
The method for forming a GaN-based semiconductor layer comprises the steps of: forming a ZnO buffer layer on one of a glass substrate and a silicon substrate; and epitaxially growing a GaN-based semiconductor layer on the ZnO buffer layer by using an electron cyclotron resonancexe2x80x94molecular beam epitaxy (ECR-MBE) method.
The method for producing a semiconductor light emitting device comprises the steps of: forming a ZnO buffer layer on one of a glass substrate and a silicon substrate; and epitaxially growing a light emitting layer made of a GaN-based semiconductor layer on the ZnO buffer layer by using a ECR-MBE method.
These methods may further comprise the step of forming an amorphous GaN-based semiconductor buffer layer on the ZnO buffer layer before the epitaxial growth step.
The epitaxial growth step is preferably performed at a temperature of 850xc2x0 C. or less and more preferably 700xc2x0 C. or less. The ZnO layer may be formed on the substrate, and the substrate is made of borosilicate and has a softening point of about 700 to 800xc2x0 C.
The GaN-based semiconductor layer or the light emitting layer may be a GaN layer or a InGaN layer.
According to the present invention, since an ECR-MBE method is used to form a GaN-based layer, it is possible for nitrogen gas to be supplied in a plasma-state by means of ECR and, by a level that is equivalent to the excitation energy occurring therein, it is possible to lower the substrate temperature.
As a result, lowering the temperature at the time of layer deposition allows the use of materials having low melting points and, accordingly, increases the selection range for the substrate materials. For example, it is difficult to use lower-priced borosilicate glass materials for the substrate in the conventional methods, but it is now possible to use such materials, thereby reducing production costs for the semiconductor light emitting device.
Also, lowering the substrate temperature prevents adverse effects resulting from differences between the thermal expansion coefficients of the substrate and GaN. In addition, in comparison with other materials, GaN and a glass substrate are closer in regard to the thermal expansion coefficient (specifically, the difference between the thermal expansion coefficients of GaN and the glass substrate is about 10%; for reference, the difference between the thermal expansion coefficients of GaN and a sapphire substrate is about 34%.) and have a higher property of ductility, cracks do not occur on the deposited GaN layer, thereby allowing a GaN layer of high quality and having high emission efficiency to be produced.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.